Encyclopedia > Iodine

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Name, Symbol, NumberIodine, I, 53
Series Halogens
Group, Period, Block17 (VIIA), 5 , p
Density, Hardness 4940 kg/m3, no data
Appearance violet-dark grey,
Atomic Properties
Atomic weight 126.90447 amu
Atomic radius (calc.) 140 (115) pm
Covalent radius 133 pm
van der Waals radius 198 pm
Electron configuration [Kr]4d10 5s2 5p5
e- 's per energy level2, 8, 18, 18, 7
Oxidation states (Oxide) 1,5,7 (strong acid)
Crystal structure Orthorhombic
Physical Properties
State of matter solid (nonmagnetic)
Melting point 386.85 K (236.66 F)
Boiling point 457.4 K (363.7 F)
Molar volume 25.72 ×10-3 m3/mol
Heat of vaporization 20.752 kJ/mol
Heat of fusion 7.824 kJ/mol
Vapor pressure __ Pa at __ K
Speed of sound __ m/s at __ K
Electronegativity 2.66 (Pauling scale)
Specific heat capacity 145 J/(kg*K)
Electrical conductivity 8.0 10-8/m ohm
Thermal conductivity 0.449 W/(m*K)
1st ionization potential 1008.4 kJ/mol
2nd ionization potential 1845.9 kJ/mol
3rd ionization potential 3180 kJ/mol
Most Stable Isotopes
isoNAhalf-life DMDE MeVDP
127I100%I is stable with 74 neutrons
131I{syn.}8.02070 dBeta-0.971131Xe
SI units & STP are used except where noted.
Iodine is a chemical element in the periodic table that has the symbol I and atomic number 53. This is an insoluble element that is required as a trace element for living organisms. Chemically, iodine is the least reactive of the halogens, and the most electropositive metallic halogen. Iodine is primarily used in medicine, photography and in dyes.

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Notable Characteristics Iodine is a bluish-black, lustrous solid that sublimes at standard temperatures into a blue-violet gas that has an irritating odor. This halogen also forms compounds with many elements, but is less active than the other member of its series and has some metallic-like properties. Iodine dissolves easily in chloroform, carbon tetrachloride, or carbon disulfide to form purple solutions (It is only slightly soluble in water). The deep blue color with starch solution is characteristic of the free element.

Applications In areas where there is little iodine in the diet - typically remote inland areas where no marine foods are eaten - iodine deficiency gives rise to goitre, so called endemic goitre. In many (but not all) such areas, this is now prevented by the addition of small amounts of sodium iodide to table salt - this product is known as iodised salt Other uses:

  • One of the halogens, it is an essential trace element; the thyroid hormones, thyroxine and triiodotyronine[?] contain atoms of iodine.
  • Tincture of iodine[?] (3% elemental iodine in water/ethanol base) is an essential component of any emergency survival kit, used both to disinfect wounds and to sanitize surface water for drinking (3 drops/L, let stand for 30 minutes.)
  • Iodine compounds are important in the field of organic chemistry and are very useful in medicine.
  • Iodides and thyroxine which contains iodine, are both used in internal medicine and, in combination with alcohol (as tincture of iodine) are used externally to disinfect wounds.
  • Potassium iodide is used in photography.

History Iodine (Gr. iodes meaning violet) was discovered by Barnard Courtois[?] in 1811.

Occurrence Iodine can be prepared in an ultrapure form through the reaction of potassium iodide with copper sulfate. There are also several other methods of isolating this element.

Isotopes There are thirty isotopes of iodine and only one, I-127, is stable. The artificial radioisotope I-131 which has a half-life of 8 days, has been used in treating cancer and other pathologies of the thyroid glands. The most common compounds of iodine are the iodides of sodium and potassium (KI) and the iodates (KIO3).

Iodine has only one stable isotope, I-127. However, radioactive isotopes of iodine have been used extensively. I-129 (half-life 17 million years) is a product of Xe-129 spallation[?] in the atmosphere, but is also the result of U-238 decay. As U-238 is produced during a number of nuclear power- related activities, its presence (as an I-129/I ratio) can indicate the type of activity going on at any one site. For this reason, I-129 was used in rainwater studies following the Chernobyl accident. It also has been used as a ground-water tracer and as an indicator of waste dispersion into the natural environment. Other applications may be hampered by the production of I-129 in the lithosphere through a number of decay mechanisms.

In many ways, I-129 is similar to Cl-36. It is a soluble halogen, fairly non-reactive, exists mainly as a non-sorbing anion, and is produced by cosmogenic, thermonuclear, and in-situ reactions. In hydrologic studies, I-129 concentrations are usually reported as the ratio of I-129 to total I (which is virtually all I-127). As is the case with Cl-36/Cl, I-129/I ratios in nature are quite small, 10-14 to 10-10 (peak thermonuclear I-129/I during the 1960s and 1970s reached about 10-7). I-129 differs from Cl-36 in that its half-life is longer (1.6 vs 0.3 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I- and iodate) which have different chemical behaviors.

Precautions Direct contact with skin can cause lesions so care needs to be taken in handling iodine. Iodine vapor is very irritating to eyes and mucous membranes. The maximum allowable concentration of iodine in air should not exceed 1 mg/m (8-hour time-weighted average - 40-hour).

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